The present invention relates to reduction of conducted electromagnetic interference (EMI) on alternating current (AC) power lines utilizing frequency domain filters of lumped parameters having significant physical structure comprising inductor devices with core(s) surrounding linear conductors.
The use of inductor-capacitor (L-C) filtering to remove conducted EMI noise in the frequency band previously mentioned is widely accepted.
Schematics of three-phase filters, such as that shown in FIG. 1, are relatively standard in the industry and it is well understood that the inductors have increasing reactance and the capacitors have decreasing reactance as frequency increases, thereby impeding and shunting the high frequency noise to minimum values.
Some basic formulas should be noted which will further the design discussions which follow: EQU I.sub.max =10*B.sub.max *A.sub.e /N*A.sub.l (1)
Where: PA1 Where: PA1 Where:
I.sub.max =Peak saturation current (A) PA2 B.sub.max =Maximum flux density (G) PA2 A.sub.e =effective core area (sq. cm) PA2 A.sub.l =inductance index (mH/1000 Turns) PA2 N=Turns EQU L=n*A.sub.l (N/1000).sup.2 (2) PA2 L=inductance (mH) PA2 n=number of magnetic cores EQU C=1/(6.28f.sub.0).sup.2 L (3) PA2 f.sub.0 =center frequency (Hz) PA2 C=capacitance (F) PA2 L=inductance (H)
The physical layout and component types in 3-phase filters usually adhere to standard filter design and construction methods which have certain disadvantages. An example of such a prior art filter is shown at FIGS. 2a and 2b, where a pair of cores 1 are shown positioned in a housing 2. FIG. 2a is a top plan view, while FIG. 2b is a cross-sectional view of the filter shown in FIG. 2a, taken generally along line 2b--2b, to have the effect of a side elevational view of the filter with the side of the housing 2 removed. In the filter shown in these figures, wires 3 are wound or wrapped around cores 1 multiple times to result in an inductor 7 having an inductance determined by the properties of the cores and the number of wraps. Capacitors 4, 5 and 6 are connected between the inductors 7 and neutral, and between the inductors and ground, to constitute a completed filter circuit as shown schematically in FIG. 1. Mylar 9 provides necessary insulation, and encapsulation material 8 provides both component mounting and insulation. Resistors 10 act as bleed resistors to bleed any residual voltage off the capacitors 4, while not affecting filter performance.
In higher current applications (180-600 A), inductors are usually constructed of several parallel turns of magnet wire wound around a medium (1-4k) permeability core. The wires themselves can be heavy and difficult to bend to form windings around brittle ferrite cores without breaking the wires, the cores, or both. The soldered, clamped or crimped connections to these inductors can also contribute to filter overheating.
Inductor and capacitor component values must be selected such that proper low pass cutoff frequencies are realized. To keep leakage current low, line-to-ground capacitors are usually kept small, thus driving up the size of the inductance, and consequently the size of the inductors.
The placement of the capacitors is driven by the inductor lead location, which often requires long leads. Leads that are too long can compromise high frequency filtering and poor location can cause high voltage isolation problems.
The present invention is directed toward relieving the aforementioned problems.